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<title>Using as: i386-Jumps</title>

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<a name="i386_002dJumps"></a>
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<p>
Next: <a href="i386_002dFloat.html#i386_002dFloat" accesskey="n" rel="next">i386-Float</a>, Previous: <a href="i386_002dMemory.html#i386_002dMemory" accesskey="p" rel="prev">i386-Memory</a>, Up: <a href="i386_002dDependent.html#i386_002dDependent" accesskey="u" rel="up">i386-Dependent</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Handling-of-Jump-Instructions"></a>
<h4 class="subsection">9.15.8 Handling of Jump Instructions</h4>

<a name="index-jump-optimization_002c-i386"></a>
<a name="index-i386-jump-optimization"></a>
<a name="index-jump-optimization_002c-x86_002d64"></a>
<a name="index-x86_002d64-jump-optimization"></a>
<p>Jump instructions are always optimized to use the smallest possible
displacements.  This is accomplished by using byte (8-bit) displacement
jumps whenever the target is sufficiently close.  If a byte displacement
is insufficient a long displacement is used.  We do not support
word (16-bit) displacement jumps in 32-bit mode (i.e. prefixing the jump
instruction with the &lsquo;<samp>data16</samp>&rsquo; instruction prefix), since the 80386
insists upon masking &lsquo;<samp>%eip</samp>&rsquo; to 16 bits after the word displacement
is added. (See also see <a href="i386_002dArch.html#i386_002dArch">i386-Arch</a>)
</p>
<p>Note that the &lsquo;<samp>jcxz</samp>&rsquo;, &lsquo;<samp>jecxz</samp>&rsquo;, &lsquo;<samp>loop</samp>&rsquo;, &lsquo;<samp>loopz</samp>&rsquo;,
&lsquo;<samp>loope</samp>&rsquo;, &lsquo;<samp>loopnz</samp>&rsquo; and &lsquo;<samp>loopne</samp>&rsquo; instructions only come in byte
displacements, so that if you use these instructions (<code>gcc</code> does
not use them) you may get an error message (and incorrect code).  The AT&amp;T
80386 assembler tries to get around this problem by expanding &lsquo;<samp>jcxz foo</samp>&rsquo;
to
</p>
<div class="smallexample">
<pre class="smallexample">         jcxz cx_zero
         jmp cx_nonzero
cx_zero: jmp foo
cx_nonzero:
</pre></div>




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